1. Field of the Invention
The present invention relates to a pulse width modulation circuit used in a write control device such as a laser beam printer or a digital copying machine.
2. Description of the Related Art
The pulse width of a pulse signal for multiple input data is modulated in such a way that a pluse having a width corresponding to the data is formed with the use of a clock signal having a frequency which is faster than that of the input data.
One type of the pulse width modulation (PWM) circuit comprises a trianglar wave generation circuit which outputs a triangular wave signal synchronizing with the pixel clock, a D/A converter circuit which converts the multiple digital input pixel data to an analogue signal, and a comparator which compares the outputs from the two circuits.
The PWM circuit functions in such a way that if the analogue input data is larger than the triangular wave signal, the comparator outputs a signals "1 ", while if the analogue input data is smaller than the triangular signal, the comparator outputs a signal "0".
In accordance with another type of the PWM circuit, a pixel clock signal is input to a plurality of delay elements which output predetermined delayed signals, respectively, to a waveform shaping circuit. A waveform selection circuit selects one delayed signal from the waveform shaping circuit in response to an input signal of pixel data and outputs a pulse signal having a width corresponding to the input data.
In accordance with the first PWM circuit mentioned above, it is necessary to prepare analogue circuits such as the D/A converter and the comparator which are very expensive especially when the speed of the pixel clock is very high such as about 10 MHz. Also, it is difficult to form the circuit as one chip of semiconductor integrated circuit.
Also, in accordance with the second PWM circuit mentioned above, since the delayed time of each element is fixed, it becomes nescessary to replace the element by another element when the frequency of the clock signal is changed, which narrows the applicability of the circuit. Besides, if the delay element is to be made from a semiconductor integrated circiut, the delay time changes due to the ambient temperature change or the fluctuation of IC process parameter, which makes it difficult to form the whole circuit as one chip of IC.